1. Field of the Invention
The present invention relates to an apparatus and a method for transmitting digital data at high speed over a transmission channel, and preferably for writing to a hard disk drive at high operating speeds.
2. Description of the Related Art
A hard disk drive uses a recording head to receive data from a device that writes, or transmits, the data. Typically, the data propagates from the writer to the head via a transmission line. The data is transmitted in the form of electrical pulses, where each electrical pulses can be characterized by a pulse width which is measured in seconds. The transmission rate of the data is an important circuit characteristic, because a higher transmission rate allows for a larger volume of information to be communicated. Therefore, a common objective of circuit designers is to increase the maximum data transmission rate. Typically, the transmitted pulse is also amplified before it is received. Thus, if there is any signal interference prior to amplification, the received signal may be distorted. Therefore, another common objective of circuit designers is to reduce or eliminate signal interference. Circuitry and transmission lines for providing high-speed communications between writing devices and receiving devices are well known in the art and are the subject of many patents.
In most cases, even when a circuit is designed to operate at a high data transmission rate, the transmission time (i.e., the length of time required for the pulse to propagate from the writer to the head) is greater than the pulse width. In these cases, there are no circuit design considerations relating to intra-pulse characteristics; the pulse is transmitted and received in an intact form. However, when a circuit is being operated at such a high speed that the transmission time is less than (or even nearly equal to) the pulse width, the circuit designer must take several intra-pulse characteristics into account, such as the rise time and fall time of the pulse and the wave propagation characteristics of the transmission line. In particular, if the voltage polarity at the writer (transmitter) changes before the previously transmitted pulse has been received at the head (receiver), then the previously transmitted pulse may be severely distorted before it is received, and the data quality may be degraded. Thus, a dilemma for writing to a hard disk drive via a high speed channel is presented.
The present invention is intended to overcome the problems noted above when the propagation time for a pulse is less than the pulse width, and provides a preamplifier writer system for a high-speed channel that preserves the integrity of transmitted pulses. In one aspect, the invention provides a circuit for writing to a read channel for a hard disk drive. The circuit may include a preamplifier writer, a non-uniform transmission line, and a head. The writer may be configured to transmit pulses having a pulse width to the head via the transmission line at a transmission speed. Each pulse may experience interference. The writer may also be configured to eliminate interference to each pulse by transmitting the pulses in a differential mode, causing the interference to be reduced. A transmission time for each pulse is inversely proportional to the transmission speed. In one aspect, the non-uniformity of the transmission line is attained by exponential broadening of a trace width of the transmission line. In another aspect, the transmission line non-uniformity is selected so that when the pulse width is greater than or approximately equal to the transmission time, the pulse propagates from the preamplifier writer to the head substantially undistorted.